1. Field of the Invention
This invention relates generally to the fabrication of magnetic transducers for disk drives and more particularly to a method of improving the reliability of the fly-height of the air-bearing-surface (ABS) of the transducer above the disk surface.
2. Description of the Related Art
In a magnetic recording disk drive there is at least one magnetic transducer which includes a read-head and a write-head incorporated within a slider. One such transducer and its associated disk is shown in (prior-art) FIG. 1a. In this figure the lower surface of the transducer (10) is co-planar with and contained within a smooth surface plane of the slider (30), called the air-bearing-surface (ABS) plane. Note that the slider is mounted on a head-gimbals assembly (50), which is shown in highly schematic form. In a disk drive containing a plurality of sliders and disks, the system shown in FIG. 1a is replicated in the vertical direction as shown in (prior-art) FIG. 1b. 
During operation of the disk drive, the slider literally flies over the surface of a rapidly spinning disk (40), suspended by air flow at a “fly-height” above the disk surface. The trailing edge of the slider (45), which is the edge past which a given portion of the disk moves last, is extremely close to the disk surface, suspended on the order of 10 nanometers (nm). The leading edge of the slider (47), which is the edge past which a given portion of the disk first moves, is significantly higher, on the order of 100 nm. Although low fly-height is desirable for high magnetic resolution of the transducer, it has distinctly negative effects on the reliability of the disk drive. Specifically, the low fly-height causes frequent and severe interference between the slider and the disk, damaging both the transducer and the ABS.
To maximize the benefit of low fly-height, the transducer is mounted very close to the trailing edge of the slider. In a typical wafer fabrication process, the transducer is located approximately 20 microns from the top of a finished wafer surface. When such a wafer is cut into sliders, the transducer is located approximately 20 microns from the trailing edge of the slider. Since the typical pitch angle (angle of the ABS relative to the disk surface) of the slider ABS is between 100 and 350 micro-radians, the fly-height difference corresponding to the 20 micron transducer-to-trailing edge distance is between 2 and 7 nm. This is a significant difference, when the desired fly-height itself is on the order of 10 mn.
Since it is relatively difficult to move the transducer closer to the trailing edge in fabrication, the trailing edge has effectively been moved closer to the transducer by the following method. In the prior art, the portion of the slider ABS between the transducer and the trailing edge of the slider is recessed by etching or other means during slider fabrication. Thus, the lowest fly-height is no longer at the actual physical edge of the slider, it is now at the edge of the recess. The recess can be made closer to the transducer by an amount on the order of 5 microns.
Referring to prior art FIG. 2a, there is shown a schematic ABS view of a transversely oriented transducer (10) emerging at the ABS plane of a slider, showing the position of the transducer relative to the transverse trailing edge (45) of the slider. The portion of the slider ABS containing the transducer ABS (20) is formed of alumina, the portion of the slider (shown foreshortened) above the transducer (13) is formed of AlTiC. The entire figure represents a single planar surface.
Referring to FIG. 2b, there is shown the basic configuration of FIG. 2a, modified by the addition of a substantially rectangular recessed (into the plane of the figure) portion (25), having two transverse edges (26) and (27). The recessed portion is shown shaded for clarity and it is a substantially planar surface, formed by etching, that lies below the planar surface of the ABS plane. One transverse edge, called herein the first edge, of the recessed portion (26) is closer to the transducer (10), than is the second transverse edge (27), which is now also the trailing edge of the slider.
Superficially, the above approach of forming a recessed region would seem to allow the transducer to fly approximately 2 nm lower (ie closer to a disk surface) without negative side effects. In fact, by recessing the ABS downstream (towards the trailing edge) from the transducer as shown above, the transducer can fly approximately 2 nm lower without increasing the mechanical stress on the disk. The mechanical stress on the transducer, however, does increase, as can be shown by the following reasoning. First, ignore the transducer and trailing edge recess and consider a planar ABS flying at a positive pitch. In a typical interference event between a slider ABS and a disk surface, whether caused by surface roughness of the disk or by a foreign particle on the disk surface, mechanical stress increases monotonically with decreasing fly-height. Thus, the stress peaks at the trailing edge of the ABS. The same is true when the ABS includes a transducer surface and when the ABS has a recessed area. Thus, at the same trailing edge fly-height, a transducer located closer to the trailing edge of the ABS experiences greater mechanical stress.
The reliability of a disk drive is affected by mechanical stress on the transducer and on the disk. The state of the art GMR read heads are particularly susceptible to mechanical stress. Thus, the recessing of the prior art is not as effective at retaining reliability as might be expected. Other, more complex ABS shapes and designs have been provided in an effort to alleviate the effects of low fly height, particularly as it relates to pitch and roll of the slider. In this regard, Bolsana et al. (U.S. Pat. No. 5,825,587) teach a slider with a support structure and etched side rails to minimize fly height during disk drive operation. Park et al. (U.S. Pat. No. 6,477,012) teach a slider ABS on which are formed a plurality of negative pressure air cavities to provide a more stable flying attitude of the slider during disk drive operation. Alexopoulos et al. teaches an ABS contact surface slider, wherein a wear-pad of negligible sixe contacts the disk surface and maintains the slider at a proper height.
The present invention does not address sliders with complex aerodynamic surface structures or sliders that are maintained at a fixed height by a disk contacting mechanism. Rather, the present invention is directed at more standard and easy to fabricate sliders with substantially ABS planar surfaces that must fly at a height maintained within given tolerances by a rigorous testing scheme.
Returning, then, to the recessed trailing edge slider of FIG. 2b, it is important to realize that, in production, the trailing edge recess only allows the mean transducer fly height to decrease by a finite amount. It does not affect the standard deviation of the fly-height distribution. It is well known that a slider with the nominal (mean) fly height will rarely fail, but the slider with lower than average fly-height will be much more likely to fail. Clearly, such probable failures should be identified and rejected.
At this time, screening the fly-height distribution in production sliders is extremely difficult. An example of an optical apparatus for measuring fly heights is provided by Wen et al. (U.S. Pat. No. 6,317,210). Optical fly-height testers are inaccurate at the fly-height range of interest of 10 nm. and below. Direct detection of interference between the slider and disk by acoustic emission, friction or readback signal modulation is meaningful only if the interference is frequent and intense. An example of an interferometric method for measuring fly height is provided by Stimiman et al. (U.S. Pat. No. 6,665,077).
Existing fly-height or interference screening methods as cited above cannot thoroughly weed out “low-flyers.” To ensure disk drive reliability at a production level the nominal (mean) fly-height must be set substantially (e.g. two standard deviations) higher than the minimum acceptable fly-height.
It is highly desirable to increase the sensitivity of interference detection methods so that the nominal fly-height can be set closer to the minimum acceptable fly-height. In that way the magnetic resolution of the transducer can be increased without sacrificing the reliability of the disk drive. The present invention provides a novel, integral, wear-pad which amplifies slider-disk interference during the product screening process, enabling low-fliers to be rejected and thereby allowing the nominal fly-height to be set at a lower level. The wear-pad wears out promptly thereafter, so that it does not itself contribute to slider-disk interference during subsequent operation of the disk drive.